Historical Introduction to the Elementary Particles 4

1.6 STRANGE PARTICLES ( ) For a brief period in 1947 it was possible to believe that the major problems of elementary particle physics were solved. After a lengthy detour in pursuit of the muon, Yukawa’s meson had finally been apprehended. Dirac’s positron had been found, and Pauli’s neutrino, although still at large (and, as we have seen, still capable of making mischief), was basically under control. The role of the muon was something of a puzzle; it seemed quite unnecessary in the overall scheme of things. On the whole, however, it looked in 1947 as though the job of elementary particle physics was essentially done.

But this comfortable state did not last long. In December of that year Rochester and Butler ” published the cloud chamber photograph shown in Figure1.8. Cosmic ray particles enter from the upper left and strike a lead plate, producing a neutral particle, whose presence is revealed when it decays into two charged secondaries, forming the upside-down “V” in the lower right. Detailed analysis shows that these charged particles are in fact. Here, then, was a new neutral particle with at least twice the mass of the pion; we call it the K 0 (“kaon”):

In 1949, Powell published the photograph reproduced in Figure1.9showing the decay of a charged kaon )The K O was first known as the and later as the ; the K + was originally called the.Their identification as neutral and charged versions of the same basic particle was not completely settled until 1956 but that’s another story, to which we shall return in Chapter 4.) The kaons behave in some respects like heavy pions, and so the meson family was extended to include them. In due course, many more mesons were discovered the, and so on.

It is some measure of the surprise with which these new heavy baryons and mesons were greeted that they came to be known collectively as “strange” particles. In 1952 the first of the modern particle accelerators (the Brookhaven Cosmotron) began operating, and soon it was possible to produce strange particles in the laboratory (before this the only source had been cosmic rays)... and with this, the rate of proliferation increased.

Not only were the new particles unexpected; there is a more technical sense in which they seemed “strange”: They are produced copiously (on a time scale of about sec), but they decay relatively slowly (typically about sec). This suggested to Pais and others that the mechanism involved in their production is entirely different from that which governs their disintegration. In modern language, the strange particles are produced by the strong force (the same one that holds the nucleus together), but they decay by the weak force (the one that accounts for beta decay and all other neutrino processes).

The details of Pais’s scheme required that the strange particles be produced in pairs. The experimental evidence for this was far from clear at that time, but in 1953 Gell- Mann and Nishijima found a beautifully simple, and, as it developed stunningly successful, way to implement and improve Pais’s idea. They assigned to each particle a new property (Gell- Mann called it “strangeness”) that (like charge, lepton number, and baryon number) is conserved in any strong interaction, but (unlike those others) is not conserved in a weak interaction. In a pion-proton collision, for example, we might produce two strange particles:

There is some arbitrariness in the assignment of strangeness numbers, obviously. We could just as well have given S = + 1 to the  ’s and the A, and S = - 1 to K + and K O ; in fact, in retrospect it would have been a little nicer that way. [In exactly the same sense, Benjamin Franklin’s original convention for plus and minus charge was perfectly arbitrary at the time, and unfortunate in retrospect since it made the current carrying particle (the electron) negative.] The significant point is that there exists a consistent assignment of strangeness numbers to all the hadrons (baryons and mesons) that accounts for the observed strong processes and “explains” why the others do not occur. (The leptons and the photon don’t experience strong forces at all, so strangeness does not apply to them.)

The garden which seemed so tidy in 1947 had grown into a jungle by 1960, and hadron physics could only be described as chaos. The plethora of strongly interacting particles was divided into two great families the baryons and the mesons and the members of each family were distinguished by charge, strangeness, and mass; but beyond that there was no rhyme or reason to it all. This predicament reminded many physicists of the situation in chemistry a century earlier, in the days before the Periodic Table, when scores of elements had been identified, but there was no underlying order or system. In 1960 the elementary particles awaited their own “Periodic Table.”